1. Field of the Invention
The present invention relates to an image forming apparatus which is applicable to printing sections of digital copying machines, and to facsimile machines, digital printers, plotters and the like, and forms an image on a recording medium by projecting a toner.
2. Description of the Related Art
In recent years, there has been proposed an image forming apparatus which converts image signals into a visible image and outputs the onto a recording medium such as paper. To give an example, Japanese Examined Patent Publication JP-A 6-30901(1994) discloses an image forming apparatus which forms a toner image directly on a recording medium such as paper without temporarily forming the toner image on a photoreceptor. The image forming apparatus is constructed in the following manner; more specifically, a particle carrier for carrying a toner, and a rear electrode facing the toner carrier, that is, a counter electrode, are arranged with an interval, and an electrode matrix, which functions as a control electrode, is interposed between the toner carrier and the counter electrode. The aforesaid image forming apparatus generates an electric field between the toner carrier and the counter electrode so that the toner is projected from the toner carrier toward the counter electrode, and controls the projection of the toner by means of the control electrode, and thus, forms a toner image directly on a recording medium provided on a side of the counter electrode which faces the toner carrier.
The control electrode is classified into a so-called single drive type and a so-called matrix drive type. The control electrode of single drive type is constructed in a manner that a plurality of gates having an aperture are formed on an insulating substrate, and a ring-like electrode is provided on an edge portion of each gate of the insulating substrate. On the other hand, the control electrode of matrix drive type is constructed in a manner that strip-like electrodes having a plurality of apertures serving as gate inlet and outlet are arranged by plural ones on both sides of an insulating substrate having a plurality of holes acting as gates, and that strip-like electrodes on both sides of the insulating substrate cross each other at right angles.
Further, an image forming apparatus having the same system as disclosed in Japanese Examined Patent Publication No. 6-30901 (1994) has been disclosed in Japanese Unexamined Patent Publication No. 4-189555 (1992). The image forming apparatus is constructed in a manner that a toner supply roller serving as a toner carrier and a base electrode serving as a counter electrode are arranged with an interval, and that toner control means serving as the single-drive type control electrode is interposed between the toner carrier and the counter electrode. The toner control means is constructed in a manner that a pair of electrodes generating an electric field for passing the toner through the gate are provided in the vicinity of each of plural gates formed on both sides of the insulating substrate, and out of the pair of electrodes, the outlet-side electrode for passing the toner through the gate is divided into two in a feeding direction of the recording medium. With the feed of the recording medium, the image forming apparatus sequentially or selectively applies a voltage between the inlet-side electrode for passing the toner through the gate and the divided electrode on an upstream side of the feeding direction of recording medium, between the inlet-side electrode and the both divided electrodes, and between the inlet-side electrode and the divided electrode on a downstream side of the feeding direction of recording medium, and then forms a toner image directly on the recording medium.
The aforesaid two Publications describe a technique of controlling the projection of the toner by the control electrode, and forming a toner image directly on a recording medium. However, the image forming apparatuses disclosed in the both Publication are different in construction and structure from the present invention, and have many problems which will be described below.
The single-drive type control electrode has a construction such that ring-like electrodes are arranged on edge portions of the plurality of gates having plural apertures in the insulating substrate. For this reason, there is required control circuit means for applying predetermined potentials to the electrodes in the vicinities of the gates corresponding to image data. The number of control circuit means must make one-to-one correspondence with respect to each gate, or more.
For example, in the image forming apparatus which longitudinally feeds a recording medium such as letter paper having a side of 8.5.times.11 inch, the resolution is 300 DPI, and the number of gates is 2560. For this reason, there is required at least 2560 high-voltage FETs for controlling a potential of each gate.
As seen from the above explanation, in order to construct the control circuit, electronic components such as a high-voltage FET, a resistor, a capacitor or the like are required for each gate; for this reason, this causes an increase in cost. Further, a power source for operating many electronic components is required; for this reason, the capacity of power source is increased. Furthermore, the control electrode is equipped with a great many FETs; for this reason, a pattern wiring of the control electrode becomes complicated, and this is a factor of making large the control electrode. As a result, there arise problems of making large the image forming apparatus, and of making complicated a method of incorporating the control electrode into the image forming apparatus or a method of replacing the control electrode. Further, in the case of trying to improve a printing resolution, there is a geometrical limit in routing a pattern wire in the vicinity of the gate. For this reason, it is difficult to provide an image forming apparatus which can form an image having high resolution.
The matrix-drive type control electrode is constructed in a manner that a plurality of upper and lower strip-like electrodes, which have plural apertures serving as gate inlets and outlets on both sides of the insulating substrate having plural holes, are arranged so as to cross each other at right angles. During printing, the upper and lower strip-like electrodes serving as inlet and an outlet of one arbitrary gate are in a state that a potential for blocking the projection of the toner toward the lower strip-like electrode (hereinafter, referred to as OFF-potential) is applied, and a potential for projecting the toner toward the upper strip-like electrode (hereinafter, referred to as ON-potential) is applied, or a state that the OFF-potential is applied to the upper strip-like electrode and the ON-potential is applied to the lower strip-like electrode. At this time, the projection of the toner is not sufficiently blocked, and then, the toner adheres onto a undesired position in an image to be formed, causing the so-called fog. As a result, a high contrast image cannot be obtained. Further, according to the construction of the control electrode, strip-like electrodes are arranged on both sides of the insulating substrate; for this reason, the thickness of the control electrode is made thicker. This increases the possibility that the toner passing through the gate is jammed in the hole. Also, a void (where the toner image is not transferred) is caused in a portion of the image formed in the latter half of the image forming process.
In the system of controlling the projection of toner by means of the control electrode and forming the toner image directly on the recording medium, a gate hole needs to have a diameter which is at least equal to a printing dot diameter or more, and in addition, ring-like electrodes must be arranged on the edge portions of the gates. In the case of providing the gates on the control electrode, for geometrical reasons, the plurality of gates must be arranged obliquely to the feeding direction of recording medium, and the toner carrier must have a sufficient width vertical to a longitudinally vertical direction. For example, it is impossible to arrange the plurality of gates in line in the longitudinally vertical direction of the toner carrier. In the case of printing a solid black image pattern, the following phenomenon occurs: even if an electric field is generated in order for the toner to pass through the gate on a portion adjacent to the projection track on the toner carrier of the toner having first passed through one of gates, there is no sufficient toner on the toner carrier; for this reason, white streaks are caused in the solid black image.
Further, the plurality of gates of the control electrode are arranged in the circumferential direction of the toner carrier, and the toner carrier has a cylindrical shape and curvature. For this reason, when the gate is separated from the nearest position to the toner carrier in the insulating substrate, a distance between the toner carrier and the gate becomes great, and then, a difference occurs in the influence of the electric field generated by ring-like electrodes on the edge portion of the gate on the toner layer of the toner carrier. For this reason, there occurs a change in the projection amount of the toner passing through the gate; as a result, inconsistencies are caused in toner density.
Since Japanese Unexamined Patent Publication JP-A 4-189555 (1992) is directed to printing an image in uniform toner density and improving graduations by providing a pair of electrodes which generate an electric field for passing the toner through a gate on both sides of a control electrode, dividing an electrode on an outlet side of the gate where the toner passed through, into two in a feeding direction of a recording medium, and further, with the feed of the recording medium, deflecting the toner which passed through the gate, in the feeding direction of the recording medium, to form a dot on the same position of the recording medium. However, even if such technique is applied to an image forming apparatus, it is impossible to solve the aforesaid problems.